The present invention relates generally to a ring topology transmission system, such as a token ring LAN (Local Area Network), and more particularly to a transmission line fault detecting method and system which are capable of easily and reliably identifying a location of a transmission line suffering from a fault with a high reliability.
In a typical ring topology transmission system, a transmission line fault is detected by using a fault locating frame having a format conforming to a standard stipulated in IEEE 802.5 (referred to as the "beacon MAC frame"). This fault locating frame has a length of approximately 40 bytes confined between a starting delimiter and an ending delimiter, wherein the frame length and intraframe format are specified by IEEE 802.5. Identifying the location of a transmission line fault is initiated when the system detects the disappearance of a token; i.e., when any token can no longer be received. Each of several individual fault detectors installed along the transmission line determines that a fault has occurred and sends out a fault locating frame. At least one of the fault detectors should fail to receive a fault detecting frame due to the fault. The system may thus determine that along the transmission line. The fault exists upstream of that detector.
Transmission line fault detecting systems such described above are disclosed in, for example, JP-A-61-187441 and JP-A-1-221954.
Known transmission line fault detecting systems provide inadequate fault detection due to their use of a fault locating frame having a predetermined length (about 40 bytes) and a predetermined format (according to IEEE 802.5). Such systems are generally incapable of detecting significant faults which allow data on the order of 40 bytes or less to be transmitted normally but which cause transmission failure on longer data strings.
Such known fault detection systems are constrained by the design of stations connected to the transmission line. In any station connected to the transmission line, only a fault locating frame according to IEEE 802.5 is regarded as an authentic fault locating frame. All frames which depart from that standard are rejected. Consequently, the fault detecting system is necessarily forced to use the fault locating frame standardized by IEEE 802.5 to ensure compatibility with the stations connected to the transmission line. In other words, the fault detecting system must use a fixed-length, standardized fault locating frame to determine the location in a transmission line at which a fault is taking place.
This constraint is problematic, since the ability to detect certain transmission line faults depends on the length of the fault locating frame used. A fault may occur in such a way that a frame of a relatively short length passes regularly through the faulty location, while a frame of a greater length is prevented from passing through. Such a fault may be undetectable with a 40-byte fault locating frame.
Thus, when the fault locating frame of the length standardized by IEEE 802.5 is sent out upon occurrence of a fault in the transmission line, an FCS (Frame Check Sequence) error may be detected with a certain probability. However, the fault may be of a type that the standard-length frame is regularly transmitted through the transmission line, thus preventing the fault detection system from locating the fault with a high degree of reliability.